The effect of cerebral ischemia and postischemia on MAO activity was investigated in Mongolian gerbils. Ischemia of 1 hour duration doesn't alter the MAO activity until 20 hours later when the reaction of MAO activity becomes apparent and persists for 1 week. In prolonged ischemia, the decrease in MAO activity is not seen prior to 5 hours of continuous unilateral common carotid artery occlusion.